Cooling fan control device and control method thereof

ABSTRACT

Disclosed herein are a cooling fan driving control device and a control method thereof non-linearly controlled to a duty ratio (%) of a PWM signal. The cooling fan driving control device includes a cooling fan control device including a temperature sensor, a microprocessor unit generating a PWM signal corresponding to a temperature measured by the temperature sensor, and a cooling fan driving unit driving the cooling fan according to a duty ratio of the PWM signal, comprising: a duty ratio detection unit receiving the PWM signal from the microprocessor unit to detect the duty ratio (%) of the PWM signal; and a duty ratio control device comparing the duty ratio (%) detected from the duty ratio detection unit with a predetermined threshold value, controlling the duty ratio (%) of the PWM signal according to the comparison results, and then, outputting the controlled PWM signal to the cooling fan driving unit.

CROSS REFERENCE(S) TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119 of Korean Patent Application Serial No. 10-2011-0100215, entitled “Cooling Fan Control Device And Control Method Thereof” filed on Sep. 30, 2011, which is hereby incorporated by reference in its entirety into this application.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a cooling fan control device and a control method thereof, and more particularly, to a cooling fan control device and a control method thereof capable of implementing a motor speed (rpm) of a cooling fan non-linearly controlled at a duty ratio (%) of a PWM signal so as to improve cooling efficiency.

2. Description of the Related Art

Various electric and electronic devices such as information processing devices such as a computer, OA devices, home appliances, power supply devices, or the like, have used a cooling fan for cooling heating generated due to operations of the devices.

Generally, revolutions, air volume, or the like, of a fan motor driving the cooling fan are defined in consideration of an expected use temperature of devices. Therefore, the devices need to be designed to have the cooling fan in order to provide a constant driving speed. In this case, cooling efficiency may be degraded or noise may occur.

Further, a method for performing a control to change a speed of the cooling fan according to a change in temperature has been used.

As described above, a technology for the method for controlling the driving speed of the cooling fan according to the change in temperature is disclosed in KR Patent Application No. 10-2004-0045419. That is, the motor speed of the cooling fan is controlled by a method of increasing the duty ratio % of the PWM signal when an object to be cooled is at high temperature and thus, the cooling fan needs to be operated at a high speed, and lowering the duty ratio % of the PWM signal when an object to be cooled is at a low temperature and thus, the cooling fan needs to be operated at low temperature.

However, the control method linearly controls the motor speed of the cooling fan according to the duty ratio (%) of the PWM signal. In this case, the control method has the following problems.

That is, when the object to be cooled is at a relatively high-temperature and thus, the motor speed of the cooling fan needs to be operated at a faster speed, the method for linearly controlling the cooling fan according to the related art is difficult to implement it.

To the contrary, when the object to be cooled is at a relatively low temperature, there is a need to operate the motor speed of the cooling fan at a slower speed so as to reduce power consumption. Likewise, the method for linearly controlling the cooling fan according to the related art is difficult to implement it.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a cooling fan control device and a control method thereof capable of controlling a motor speed (rpm) of the cooling fan non-linearly controlled at a duty ratio (%) of a PWM signal.

According to an exemplary embodiment of the present invention, there is provided a cooling. fan control device including a temperature sensor, a microprocessor unit generating a PWM signal corresponding to a temperature measured by the temperature sensor, and a cooling fan driving unit driving the cooling fan according to a duty ratio of the PWM signal, including: a duty ratio detection unit receiving the PWM signal from the microprocessor unit to detect the duty ratio (%) of the PWM signal; and a duty ratio control device comparing the duty ratio (%) detected from the duty ratio detection unit with a predetermined threshold value, controlling the duty ratio (%) of the PWM signal according to the comparison results, and then, outputting the controlled PWM signal to the cooling fan driving unit.

The duty ratio control device may control the duty ratio (%) of the PWM signal by comparing the duty ratio (%) detected from the duty ratio detection unit with the predetermined threshold value and adding and subtracting the predetermined duty ratio control value to and from the detected duty ratio (%) of the PWM signal according to the comparison results.

The duty control device may add the duty ratio control value to the detected duty ratio (%) of the PWM signal when the duty ratio (%) detected from the duty ratio detection unit is a predetermined first threshold value or more; and subtract the duty ratio control value from the detected duty ratio (%) of the PWM signal when the duty ratio (%) detected from the duty ratio detection unit is a predetermined second threshold value or less, the first threshold value being larger than the second threshold value.

The first threshold value may be any one of 70% to 90%, the second threshold value may be any one of 10% to 30%, and the duty ratio control value may be any one of 1% to 10%.

The cooling fan control device may further include a PWM signal generation unit generating a new PWM signal by receiving the PWM signal from the duty ratio control device and changing a frequency of the PWM signal according to a predetermined frequency value.

The PWM signal generation unit may perform a function of setting the duty ratio (%) of the PWM signal to a lowest threshold value when the duty ratio (%) of the PWM signal received from the duty ratio control device is a predetermined lowest threshold value or less.

According to an exemplary embodiment of the present invention, there is provided a cooling fan speed control method, including: (a) receiving a PWM signal generated corresponding to temperature measured by a temperature sensor; (b) detecting a duty ratio (%) of the PWM signal; (c) comparing the duty ratio (%) detected at the detecting of the duty ratio with a predetermined first threshold value; (d) when the duty ratio (%) detected at the comparing of the duty ratio is below the predetermined first threshold value, comparing it with the predetermined second threshold value; and (e) generating a new PWM signal by changing the frequency of the PWM signal according to the predetermined frequency value when the duty ratio (%) exceeds the predetermined second threshold value at the comparing of the duty ratio (%), wherein the first threshold value is larger than the second threshold value.

The cooling fan speed control method may further include at the comparing of the duty ratio, when the detected duty ratio (%) is the predetermined first threshold value or more, adding the predetermined duty ratio control value to the detected duty ratio (%) of the PWM signal; and generating a new PWM signal by changing a frequency of the controlled PWM signal according to a predetermined frequency value.

The cooling fan speed control method may further include: at the comparing, when the detected duty ratio (%) is the predetermined second threshold value or less, subtracting the predetermined duty ratio control value from the detected duty ratio (%) of the PWM signal; comparing the controlled duty ratio (%) of the PWM signal with a lowest threshold value; and generating a new PWM signal by changing the fixed frequency of the PWM signal according to the predetermined frequency value when the controlled duty ratio (%) of the PWM signal exceeds the lowest threshold value.

The cooling fan speed control method may further include: fixing the controlled duty ratio (%) of the PWM signal to a lowest threshold value when the controlled duty ratio (%) of the PWM signal is the lowest threshold value; and generating the new PWM signal by changing the controlled frequency of the PWM signal according to the predetermined frequency value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an entire power measurement system according to an exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a waveform of a PWM signal.

FIG. 3A is a graph between a non-controlled duty ratio (%) of the PWM signal and a motor speed (rpm) driving a cooling fan.

FIG. 3B is a graph between a controlled duty ratio (%) of the PWM signal and a motor speed (rpm) driving a cooling fan.

FIG. 3C is a graph coupling FIGS. 3A and 3B.

FIG. 4 is a flow chart of the method for controlling a cooling fan according to the exemplary embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various advantages and features of the present invention and methods accomplishing thereof will become apparent from the following description of embodiments with reference to the accompanying drawings. However, the present invention may be modified in many different forms and it should not be limited to the embodiments set forth herein. These embodiments may be provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals throughout the description denote like elements.

In addition, terms used in the present specification are for explaining the embodiments rather than limiting the present invention. Unless explicitly described to the contrary, a singular form includes a plural form in the present specification. The word “comprise” and variations such as “comprises” or “comprising,” will be understood to imply the inclusion of stated constituents, steps, operations and/or elements but not the exclusion of any other constituents, steps, operations and/or elements.

Hereinafter, a configuration and an acting effect of exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

FIG. 1 shows a block diagram of an entire cooling fan control device 100 according to an exemplary embodiment of the present invention.

Referring to FIG. 1, according to the exemplary embodiment of the present invention, a cooling fan control device 100 including a temperature sensor (not shown), a microprocessor unit (not shown) generating a PWM signal corresponding to temperature measured in the temperature sensor, and a cooling fan driving unit (not shown) driving the cooling fan according to a duty ratio of the PWM signal may include a duty ratio detection unit 110 receiving the PWM signal from the microprocessor unit to detect the duty ratio (%) of the PWM signal; and a duty ratio control device 120 comparing the duty ratio (%) detected from the duty ratio detection unit with a predetermined threshold value, controlling the duty ratio (%) of the PWM signal according to the comparison results, and then, outputting the controlled PWM signal to the cooling fan driving unit.

Generally, a cooling fan control system first measures the temperature of an object to be cooled by attaching the temperature sensor to an object to be cooled. The temperature sensor may use both an analog temperature sensor and a digital temperature sensor, wherein as the analog temperature sensor, a thermister is generally used. The thermister, which uses a characteristic in which resistivity is changed in response to the change in temperature, is connected to a dividing resistor in series. In this case, the temperature voltage may be calculated by a resistance changing value of the thermister and the dividing voltage of the dividing resistor.

Next, the PWM signal corresponding the measured temperature is generated in a microprocessor unit (MPU) (not shown) using the signal output from the temperature sensor.

FIG. 2 is a diagram showing a waveform of the PWM signal output from the microprocessor unit (MPU). The PWM signal may set a high region to be on duty and may also set a low region to be on duty. Herein, for convenience of explanation, the high region is set to be on duty.

When the temperature of the object to be cooled is low and thus, the low-speed rotation of the cooling fan is required, the cooling fan may be rotated at a low speed by shortening the on duty value (%) of the PWM signal shown in FIG. 2.

Meanwhile, when the temperature of the object to be cooled is high and thus, the high-speed rotation of the cooling fan is required, the cooling fan may be rotated at a high speed by extending the period of the on duty value (%) of the PWM signal.

As described above, the microprocessor unit (MPU) may generate the PWM signal by controlling the ratio of on-off time in a period corresponding to the temperature measured by the temperature sensor, that is, the duty ratio (%).

The duty ratio detection unit 110 may serve a function of receiving the PWM signal generated corresponding to the temperature in the microprocessor unit (MPU) to detect the duty ratio (%) of the received PWM signal.

The duty ratio (%) of the PWM signal may be detected using a pulse edge detection circuit configured to include an integrator including a resistor and a capacitor connected in parallel, a delayer including a Schmitt trigger inverter circuit and an inverter to receive and delay the pulse signal of the integrator, a phase comparator including an exclusive-Or gate logic element to compare the pulse signal of the delayer with the phase of the input pulse signal, or the like. In addition, the duty ratio (%) of the PWM signal may be detected by using several components generally well known in the art to which the present invention pertains such as a technology disclosed in KR Patent Laid-Open No. 1998-082412, or the like.

The duty ratio control device 120 may perform a function of receiving the data value relating to the duty ratio (%) of the PWM signal detected from the duty ratio detection unit 110 to compare the received data value with the predetermined threshold value to control the duty ratio (%) of the PWM signal according to the result value.

Describing in more detail, the duty ratio control device 120 compares the duty ratio (%) detected from the duty ratio detection unit with the predetermined threshold value and adds and subtracts the predetermined duty ratio control value to and from the duty ratio (%) of the detected PWM signal according to the comparison results to control the duty ratio (%) of the PWM signal.

Here, a duty ratio control value of the threshold value and the duty ratio control device may be determined as the duty ratio (%) value having a predetermined value.

Meanwhile, although not shown in the drawings, the duty ratio control device 120 may include a memory, or the like, that may store the threshold value, the comparison result value, or the like.

Now, in the duty ratio control device 120, a method for controlling the duty ratio (%) of the PWM signal detected by the duty ratio detection unit 110 will be described in more detail.

When the duty ratio control device 120 receives the data value relating to the duty ratio (%) of the PWM signal from the duty ratio detection unit 110, the duty ratio control device 120 compares the received data value with a first threshold value and when the duty ratio (%) of the PWM signal is the first threshold value or more, the duty ratio control device 120 adds the duty ratio control value to the detected duty ratio of the PWM signal.

To the contrary, when the duty ratio (%) detected from the duty ratio detection unit 110 is the predetermined second threshold value or less, the duty ratio control value is subtracted from the detected duty ratio (%) of the PWM signal.

In this case, the first threshold value may be set to be a value larger than the second threshold value. Therefore, when the duty ratio (%) of the PWM signal detected from the duty ratio detection unit 110 is less than the first threshold value and the second threshold value exceeds, the duty ratio control value is not separately added and subtracted.

For example, if it is assumed that the first threshold valued is 70%, the second threshold value is 30%, and the duty ratio control value is 5%, when the duty ratio (%) of the PWM signal detected from the duty ratio detection unit 110 is 30%, the duty ratio control device 120 controls the duty ratio (%) of the PWM signal to 25% and outputs the controlled duty ratio and when the duty ratio (%) of the PWM signal detected from the duty ratio detection unit 110 is 70%, the duty ratio control device 120 uses a method of controlling and outputting the duty ratio (%) of the PWM signal to 75%. Further, when the detected duty ratio (%) of the PWM signal from the duty ratio detection unit 110 is above 30% to below 70%, the duty ratio detection unit 110 outputs the duty ratio of the PWM signal as it is without performing a control.

The PWM signal passing through the duty ratio control device 120 may be transferred to a motor unit (not shown) driving the cooling fan and drive the cooling fan according to the duty ratio (%) of the PWM signal.

The driving speed of the cooling fan driven by the PWM signal controlled by the duty ratio control device 120 will be described below.

FIG. 3 shows a graph between the duty ratio (%) of the PWM signal and the motor speed (rpm) driving the cooling fan, wherein FIG. 3A shows a graph between the non-controlled duty ratio (%) of the PWM signal and the motor speed (rpm) driving the cooling fan and FIG. 3B shows the graph between the duty ratio (%) of the PWM signal controlled by the duty ratio control device 120 and the motor speed (rpm) driving the cooling fan.

As shown in FIG. 3A, when the separate control is not performed, the motor speed (rpm) driving the cooling fan is linearly proportional to the duty ratio (%) of the PWM signal. On the other hand, when the duty ratio (%) of the PWM signal is controlled by the duty ratio control device 120, as shown in FIG. 3B, the motor speed (rpm) corresponding to the duty ratio (%) to which the duty ratio control value is added in a period (a) of duty ratio (%) (1) or more corresponding to the first threshold value is shown and the motor speed (rpm) corresponding to the duty ratio (%) from which the duty ratio control value is subtracted in a period (b) of duty ratio (%) (2) or less corresponding to the second threshold value is shown. Meanwhile, since the duty ratio control value is separately added and subtracted in a period (c) exceeding the duty ratio (%) (2) corresponding to the second threshold value and below the duty ratio (%) (1) corresponding to the first threshold value, the motor speed (rpm) is shown similar to FIG. 3A.

According to the above-mentioned operation, the object to be cooled may be more efficiently cooled. That is, FIG. 3C is a graph coupling FIGS. 3A and 3B. As shown in FIG. 3C, when the object to be cooled is at a relatively high temperature and thus, the duty ratio (%) of the PWM signal enters period (a) of FIG. 3C, the cooling fan control device 100 according to the embodiment of the present invention may drive the motor speed (rpm) of the cooling fan at a high speed by a difference of period (d), as compared with the cooling fan control device according to the related art linearly increasing.

To the contrary, when the object to be cooled is at a relatively low temperature and thus, the duty ratio (%) of the PWM signal enters period (b) of FIG. 3C, the cooling fan control device 100 according to the embodiment of the present invention may drive the motor speed (rpm) of the cooling fan at a low speed by a difference of period (e), as compared with the cooling fan control device according to the related art linearly reduced, thereby reducing the power consumption of the cooling fan control device.

Herein, periods (d) and (e) may be increased or reduced by controlling the duty ratio control value. That is, the motor speed (rpm) may be controlled in periods (a) and (b).

Further, period (a) or (b) may be increased or reduced by controlling the first threshold value and the second threshold value.

Therefore, the duty ratio control value and the first threshold value and the second threshold value may be appropriately selected in consideration of the characteristics and the peripheral environment of the object to be cooled.

Generally, the first threshold value may be set to be any one in the range of 70% to 90%, the second threshold value may be set to be any one in the range of 10% to 30%, and the duty ratio control value may be set to be any one in the range of 1% to 10%. However, this is a general numerical range for helping understanding and therefore, it is to be noted that the exemplary embodiment of the present invention is not necessarily limited to the above-mentioned numerical range.

Meanwhile, the cooling fan control device 100 according to the exemplary embodiments of the present invention receives the PWM signal from the duty ratio control device 120 and may further include a PWM signal generation unit 130 that changes the frequency of the PWM signal according to the predetermined frequency value to generate a new PWM signal.

The frequency of the PWM signal generated corresponding to the temperature measured by the temperature sensor may be selected in a wide range between, for example, 100 Hz tO 100 KHz according to a microprocessor unit (MPU) and thus, the PWM signal generation unit 130 changes the frequency of the PWM signal output from the duty ratio control device 120 to the predetermined frequency value (generally, 25 KHz) in consideration of an audible frequency 20 to 20000 Hz of human to generate the new PWM signal.

However, when the frequency of the PWM signal is too high, the duty loss may be increased. Therefore, it is preferable to appropriately select the frequency value to be changed.

In addition, the PWM signal generation unit 130 may serve to fix the duty ratio (%) of the PWM signal to the lowest threshold value when the duty ratio (%) of the PWM signal input from the duty ratio control device 120 is the predetermined lowest threshold value or less.

That is, if the object to be cooled is a product using current, the driving of the cooling fan needs to be minimally secured except for the special situations. Therefore, the PWM signal generation unit 130 sets the lowest threshold value and when the duty ratio (%) of the PWM signal input from the duty ratio control device 120 is the predetermined lowest threshold value or less, that is, the case of period (f) of FIG. 3B, the cooling fan may be operated at a minimum mode by fixing the duty ratio of the PWM signal to the lowest threshold value.

Therefore, the lowest threshold value may be set to be lower than the second threshold value. As a result, the lowest threshold value needs to be set in consideration of the duty ratio (%) in which the duty ratio control value is subtracted from the duty ratio control device 120.

A cooling fan control method according to the cooling fan control device 100 of the present invention will be described below.

FIG. 4 is a flow chart of the method for controlling a cooling fan according to the exemplary embodiment of the present invention. Referring to FIG. 4, the cooling fan control method according to the exemplary embodiment of the present invention may first perform receiving the PWM signal generated corresponding to the temperature measured by the temperature sensor in the duty ratio detection unit 110 (S10).

When the PWM signal is input, the duty ratio detection unit 110 may perform detecting the duty ratio (%) of the input PWM signal (S20).

When the duty ratio (%) of the PWM signal is detected according to step S20, the duty ratio detection unit 110 outputs the data value relating to the detected duty ratio (%) to the duty ratio control device 120 and the duty ratio control device 120 may perform comparing the data value with the predetermined first threshold value (S30).

When the detected duty ratio (%) is below the predetermined first threshold value, the duty ratio control device 120 may perform again comparing it with the predetermined second threshold value (S40).

At step S40, when the detected duty ratio (%) exceeds the predetermined second threshold value, the duty ratio control device 120 does not perform the separate control for the PWM signal detected according step S20 and outputs it to the PWM signal generation unit 130.

The PWM signal generation unit 130 performs generating the new PWM signal by changing the frequency of the input PWM signal according to the predetermined frequency value (S50) and may drive the cooling fan by transferring the new PWM signal to the motor unit (not shown) driving the cooling fan.

Meanwhile, at step S30, when the detected duty ratio (%) is the predetermined first threshold value or more, the duty ratio control device 120 performs adding the predetermined duty ratio control value to the detected duty ratio (%) of the PWM signal (S60) and outputs the controlled PWM signal to the PWM signal generation unit 130.

Then, the PWM signal generation unit 130 generates the new PWM signal of which the frequency is changed according the predetermined frequency value through step S50 and may drive the cooling fan by transferring the new PWM signal to the motor unit (not shown) driving the cooling fan. Therefore, when the object to be cooled is at a relatively high temperature, the object to be cooled may be more efficiently cooled than the cooling speed control method according to the related art.

Further, at step S40, when the detected duty ratio (%) is the predetermined second threshold value or less, the duty ratio control device 120 performs subtracting the predetermined duty ratio control value from the detected duty ratio (%) of the PWM signal (S70) and then, outputs the controlled PWM signal to the PWM signal generation unit 130.

The PWM signal generation unit 130 performs comparing the controlled duty ratio (%) of the PWM signal with the predetermined lowest threshold value (S80) and when the controlled duty ratio (%) of the PWM signal exceeds the lowest threshold value, is branched into step S50 to drive the cooling fan. Therefore, when the objected to be cooled is at the relatively low temperature, the power consumption of the cooling fan control device can be reduced as compared with the cooling fan control method according to the related art.

In order to secure the lowest driving of the cooling fan, when the controlled duty ratio (%) of the PWM signal is the lowest threshold value at step S80, fixing the controlled duty ratio (%) of the PWM signal to the lowest threshold value is performed (S90) and is branched to step S50, thereby driving the cooling fan.

As set forth above, the cooling fan control device and the control method thereof the exemplary embodiments of the present invention can operate the motor speed (rpm) of the cooling fan at faster speed than the cooling fan control device linearly increased and the control method thereof according to the related art when the object to be cooled is at the relatively high temperature, thereby improving the cooling efficiency.

In addition, the exemplary embodiments of the present invention can operate the motor speed of the cooling fan at a slower speed than the cooling fan control device linearly increased and the control method thereof according to the related art when the object to be cooled is at the relatively low temperature, thereby reducing the power consumption of the cooling fan control device.

The present invention has been described in connection with what is presently considered to be practical exemplary embodiments. Although the exemplary embodiments of the present invention have been described, the present invention may be also used in various other combinations, modifications and environments. In other words, the present invention may be changed or modified within the range of concept of the invention disclosed in the specification, the range equivalent to the disclosure and/or the range of the technology or knowledge in the field to which the present invention pertains. The exemplary embodiments described above have been provided to explain the best state in carrying out the present invention. Therefore, they may be carried out in other states known to the field to which the present invention pertains in using other inventions such as the present invention and also be modified in various forms required in specific application fields and usages of the invention. Therefore, it is to be understood that the invention is not limited to the disclosed embodiments. It is to be understood that other embodiments are also included within the spirit and scope of the appended claims. 

What is claimed is:
 1. A cooling fan control device including a temperature sensor, a microprocessor unit generating a PWM signal corresponding to a temperature measured by the temperature sensor, and a cooling fan driving unit driving the cooling fan according to a duty ratio of the PWM signal, comprising: a duty ratio detection unit receiving the PWM signal from the microprocessor unit to detect the duty ratio (%) of the PWM signal; and a duty ratio control device comparing the duty ratio (%) detected from the duty ratio detection unit with a predetermined threshold value, controlling the duty ratio (%) of the PWM signal according to the comparison results, and then, outputting the controlled PWM signal to the cooling fan driving unit.
 2. The cooling fan control device according to claim 1, wherein the duty ratio control device controls the duty ratio (%) of the PWM signal by comparing the duty ratio (%) detected from the duty ratio detection unit with the predetermined threshold value and adding and subtracting the predetermined duty ratio control value to and from the detected duty ratio (%) of the PWM signal according to the comparison results.
 3. The cooling fan control device according to claim 1, wherein the duty control device adds the duty ratio control value to the detected duty ratio (%) of the PWM signal when the duty ratio (%) detected from the duty ratio detection unit is a predetermined first threshold value or more; and subtracts the duty ratio control value from the detected duty ratio (%) of the PWM signal when the duty ratio (%) detected from the duty ratio detection unit is a predetermined second threshold value or less, the first threshold value being larger than the second threshold value.
 4. The cooling fan control device according to claim 3, wherein the first threshold value is any one of 70% to 90%, the second threshold value is any one of 10% to 30%, and the duty ratio control value is any one of 1% to 10%.
 5. The cooling fan control device according to claim 1, further comprising a PWM signal generation unit generating a new PWM signal by receiving the PWM signal from the duty ratio control device and changing a frequency of the PWM signal according to a predetermined frequency value.
 6. The cooling fan control device according to claim 5, wherein the PWM signal generation unit performs a function of setting the duty ratio (%) of the PWM signal to a lowest threshold value when the duty ratio (%) of the PWM signal received from the duty ratio control device is a predetermined lowest threshold value or less.
 7. A cooling fan speed control method, comprising: (a) receiving a PWM signal generated corresponding to temperature measured by a temperature sensor; (b) detecting a duty ratio (%) of the PWM signal; (c) comparing the duty ratio (%) detected at the detecting of the duty ratio with a predetermined first threshold value; (d) when the duty ratio (%) detected at the comparing of the duty ratio is below the predetermined first threshold value, comparing it with the predetermined second threshold value; and (e) generating a new PWM signal by changing the frequency of the PWM signal according to the predetermined frequency value when the duty ratio (%) exceeds the predetermined second threshold value at the comparing of the duty ratio (%), wherein the first threshold value is larger than the second threshold value.
 8. The cooling fan speed control method according to claim 7, further comprising: at the comparing of the duty ratio, when the detected duty ratio (%) is the predetermined first threshold value or more, adding the predetermined duty ratio control value to the detected duty ratio (%) of the PWM signal; and generating a new PWM signal by changing a frequency of the controlled PWM signal according to a predetermined frequency value.
 9. The cooling fan speed control method according to claim 7, further comprising: at the comparing, when the detected duty ratio (%) is the predetermined second threshold value or less, subtracting the predetermined duty ratio control value from the detected duty ratio (%) of the PWM signal; comparing the controlled duty ratio (%) of the PWM signal with a lowest threshold value; and generating a new PWM signal by changing the fixed frequency of the PWM signal according to the predetermined frequency value when the controlled duty ratio (%) of the PWM signal exceeds the lowest threshold value.
 10. The cooling fan speed control method according to claim 9, further comprising: fixing the controlled duty ratio (%) of the PWM signal to a lowest threshold value when the controlled duty ratio (%) of the PWM signal is the lowest threshold value; and generating the new PWM signal by changing the controlled frequency of the PWM signal according to the predetermined frequency value. 